Three-dimensional displays can be of several forms. Those such as holographic displays that form an exact optical representation of three-dimensional objects through phase and amplitude modulation of light. Others recreate three-dimensional information using volume displays such as a series of synchronized modulating two-dimensional screens. Although, these approaches more closely reproduce true three-dimensional images, they are very demanding of hardware and at present can only form very crude images. A more practical approach is to form stereoscopic images in which one image is seen only by the right eye and a second image by the left. The difference between the images yields depth information, thereby providing a strong three-dimensional sensation, whereby objects appear to be a few meters away from a viewer in a cinema environment.
Conventionally, stereoscopic images are viewed through eyewear that discriminates between the eyes. Eyewear can discriminate through color as used in so-called anaglyph stereo systems. One eye can be made to see one portion of the visible spectrum while the other eye sees a complementary portion of the spectrum. Encoding the stereoscopic images in the same color bands can yield a three-dimensional sensation although the observable difference in what the eyes see causes fatigue.
Contrasted to color-based left/right stereoscopic discrimination, an alternative method of eyewear discrimination is to use polarization. One eye can be made to see one polarization and the other its orthogonal counterpart by making eyewear with lenses made from orthogonally aligned linear polarizers. Though less fatiguing to the eye than anaglyph eyewear, linear polarization states demand restriction on the orientation of the viewer's head. Another polarization-based solution is to use orthogonal left and right circularly polarized light for the two stereo image channels, thereby reducing the orientation constraints of the viewer's head.
Stereoscopic systems that encode separately left and right eye information traditionally use two projectors or spatially interlaced direct view displays. A more attractive approach uses a single display with an optical modulator allowing alternate frames to be viewed by different eyes. Shutter glasses that have liquid crystal modulating lenses can discriminate temporally and work well with a single fast display such as a conventional CRT. Passive eyewear with polarization modulation of left and light eye images from a single fast projector is preferred however for large projected images with multiple viewers.
A known approach to 3D projection involves the polarization switch (z-screen), which is chromatic in performance, and has been described in detail in U.S. Pat. No. 4,792,850 issued Dec. 20, 1988 to Lipton et al. In a known cinema system using the teachings of Lipton, a high frame rate (>100 Hz) three-chip (RGB) DLP projector creates alternate left and right eye images in synchronization with the z-screen, which creates substantially circular polarized states, but exhibits significant chromatic performance. Furthermore the eyewear has to be of a matching circularly polarized form adding cost to a presentation relative to a linear polarized system.
FIG. 1a illustrates Lipton's z-screen switch 10, which consists of paired nematic liquid crystal (LC) quarter wave switches 14, 16 oriented at 90° to each other and at 45° to the required input polarizer 12. The Z-screen switch 10 is used with passive circular polarized eyewear for stereo projection.
In one state, where a low voltage is applied to a first LC cell 14 and a high voltage to a second LC cell 16, the z-screen 10 creates left handed circularly polarized output light for a specific design wavelength, typically 550 nm. By swapping voltages, right handed polarization is produced. By making the analyzing circular polarizing (CP) eyewear matched to the z-screen 10 and aligned at the correct orientation angle, it is possible to create near perfect chromatic blocking for the viewer. That is, the right eye image is solely seen by the right eye with no contamination or cross-talk from the image destined for the left, and vice versa. However, under this condition the correct right eye image is deficient of red and blue light when compared to the original image requiring color balance and associated light loss. Furthermore, chromatic behavior is seen when the eyewear is oriented such as when the viewer tilts his or her head. Although the circularly encoded polarization state minimizes cross-talk as a function of head tilt (and indeed perfectly suppresses it for the light around 550 nm for which it is designed), magenta light is seen to contaminate at a level that can be noticeable under certain conditions.
FIG. 1b is a graph 20 showing the relation of leakage intensity to wavelength of z-screen modulated light. Indeed, the extent of the chromatic performance of the incumbent z-screen 10 can be illustrated by analyzing the output with an ideal achromatic circular polarizer. For blue and red light wavelengths either side of 520 nm, the polarization states are elliptical leading to a chromatic performance. Such chromatic behavior that is wavelength-dependent and influenced by head tilt is undesirable as it affects the viewing experience.